The infrastructure of modular computer systems has had to adapt to these new conditions and one of the most significant developments has been the move from parallel architecture to serial bus technology using fast data transfer protocols.
The 'old' parallel open platform standards now have serial extensions based on the traditional Eurocard format, whilst newer bus technologies – such as ATCA and MicroTCA – have employed serial techniques from the outset, albeit with a different form factor that eliminates backward compatibility.
In future, as bus drivers and cpu chipsets become unavailable or reach end of life status, designers who used VME or CompactPCI will be forced to choose a new technology
VMEbus and its descendants
VMEbus was the first open platform standard, mainly promoted by Motorola with the 68000 generation of processors. In 1994 its data transfer rate was increased from 32bit to 64bit by VITA (the VMEbus International Trade Association). While the classic parallel VMEbus is of decreasing importance, many systems and backplanes based on the VME64x extension are still in use.
The next step was VXS, which provided the serial data transfer capability required for today's high speed systems. This specification included a set of protocol layer standards – including RapidIO, Ethernet and InfiniBand – to define the specific serial interconnect used in a system implementation. While backward compatibility with VME64 extensions was assured, the more recent serial specification, VPX, uses entirely new high speed connectors, so VPX and VME64x backplanes are no longer compatible.
CompactPCI
In the mid 1990s, PICMG developed the CompactPCI specification as a cost effective open platform standard for high end 19in systems. Featuring rugged 3U or 6U Eurocard formats and twice as many slots as the standard desktop PCI specification, CompactPCI offered a packaging scheme better suited to use in industrial systems. However, since CompactPCI provides no serial interfaces via the backplane, the specification was extended in 2005 with CompactPCI Express, which uses serial, rather than parallel, data transmission. For many users, this new specification was 'too open', making it difficult for them to create systems with standardised backplanes. Furthermore, in CompactPCI Express the number of rear I/O connections was too small and it lacked important features, such as support for S-ATA, USB and Ethernet.
PICMG has developed a new backplane standard – CompactPCI Serial – which addresses these perceived shortcomings and offers data transfer speeds of up to 32Gbit/s. Though only finalised in March 2011, CompactPCI Serial systems, backplanes and boards are now available.
The main focus for CompactPCI Serial is on applications in industrial automation and transportation. The standard can be used not only for simple machine control systems, but also for applications that require large amounts of data to be processed: for example, video capture and analysis. In transportation engineering, CompactPCI Serial is suitable for control systems on trains, passenger information and entertainment systems both on trains and at stations, signal box control systems and trackside measurement systems.
ATCA: serial from the start
In 2001, PICMG published the AdvancedTCA specification to provide the first unified standard for modular computer systems in the telecoms industry. This defined all system aspects, including mechanics, backplane and cooling. It also defined management – the controlled startup, shutdown and monitoring of boards. In conceptual terms, ATCA is a scalable, high performance architecture with high function density and 99.999% availability. This makes it particularly suitable for server applications in the core network, where large volumes of data need to be processed.
An important innovation was the change from parallel data buses to fast serial point to point connections, which made it possible to send common protocols (such as Ethernet, RapidIO and PCI Express) via the backplane.
The basic building blocks of an ATCA system are the ATCA board or 'blade' and the AdvancedMC carrier, which may house up to eight Advanced Mezzanine Card (AdvancedMC) modules. Several blades and carriers can be assembled into a subrack and, in a typical system, populated subracks are installed within a 19in cabinet. To ensure the ATCA blades provided sufficient space to accommodate all the necessary components and to allow them to be cooled effectively, the form factor was enlarged in comparison to Eurocards: 8U high, 6HP wide and 280mm deep. In addition, the specification defined the 70mm deep rear transmission module, through which, typically, a large number of data sources would be linked to the system.
Unlike previous generations of sub assemblies, such as the PMC boards used in VME and CompactPCI systems, AdvancedMC modules were designed to be hot swappable and controlled from the shelf management system. This, coupled with redundancy of key sub assemblies, ensured a minimum availability of 99.999%. This means a system is only allowed to be inoperable for 5.3 minutes per year.
Small form factor, high performance
Following ATCA, PICMG developed another modular standard called MicroTCA, which focuses on cost sensitive, physically smaller applications with reduced capacity and performance and possibly less stringent availability requirements. In the ATCA standard, AdvancedMC modules are inserted into AdvancedMC carriers; in MicroTCA, these sub-assemblies are defined as functional modules that are used directly in the system. MicroTCA therefore allows compact, powerful computer systems to be built.
MicroTCA is being used in a number of non telecoms applications, where large volumes of data need to be processed, including real time motion control, industrial and medical image processing and test and measurement systems.
Recognising MicroTCA's broader appeal, PICMG set up four MicroTCA sub specifications to address the requirements of applications outside the telecoms sector. The first of these, MicroTCA.1, defines ruggedised, air cooled systems for demanding mechanical and thermal conditions. MicroTCA.1 products are already in use in the field. The second sub specification, MicroTCA.2, is concerned with air cooled, robust MicroTCA systems for military applications and is still in development. The third, MicroTCA.3, deals with conduction cooled, robust MicroTCA systems for military applications; this sub-specification has already been finalised.
A further area of application outside telecoms is described by in MicroTCA.4 as 'Enhancements for Rear I/O and Precision Timing'. This defines additions and modifications to make MicroTCA useful for the physics research community. Systems of this type will be used, for example, to control particle accelerators for high energy physics experiments. However, MicroTCA.4 is also suitable for other segments, including test and measurement.
Conclusion
The parallel bus in open platform standards has been replaced by the faster serial data transmission model. Both technologies are, however, sure to coexist for another five to seven years. Which bus technology and which degree of migration is best for each particular area of application should always be carefully considered, including the cost aspects, before any general technology migration is undertaken.
Author profiles:
Martin Traut is product manager at Schroff GmbH. Keith Reynolds is technical/marketing manager at Schroff UK.
